Control of One-Handed Helicity in Polyacetylenes: Impact of an Extremely Small Amount of Chiral Substituents.

Controlling the one-handed helicity in synthetic polymers is crucial for developing helical polymer-based advanced chiral materials. We now report that an extremely small amount of chiral biphenylylacetylene (BPA) monomers (ca. 0.3-0.5 mol %) allows complete control of the one-handed helicity throughout the polymer chains mostly composed of achiral BPAs. Chiral substituents introduced at the 2-position of the biphenyl units of BPA positioned in the vicinity of the polymer backbones contribute to a significant amplification of the helical bias, as interpreted by theoretical modeling and simulation. The helical structures, such as the helical pitch and absolute helical handedness (right- or left-handed helix) of the one-handed helical copolymers, were unambiguously determined by high-resolution atomic force microscopy combined with X-ray diffraction. The exceptionally strong helix-biasing power of the chiral BPA provides a highly durable and practically useful chiral material for the separation of enantiomers in chromatography by copolymerization of an achiral functional BPA with a small amount of the chiral BPA (0.5 mol %) due to the robust helical scaffold of the one-handed helical copolymer.

Emergence of Highly Enantioselective Catalytic Activity in a Helical Polymer Mediated by Deracemization of Racemic Pendants.

Any polymers composed of racemic repeating units are obviously optically inactive and hence chiral functions, such as asymmetric catalysis, will not be expected at all. Contrary to such a preconceived notion, we report an unprecedented helical polymer-based highly enantioselective organocatalyst prepared by polymerization of a racemic monomer with no catalytic activity. Both the right- and left-handed helical poly(biarylylacetylene)s (PBAs) composed of dynamically racemic 2-arylpyridyl-N-oxide monomer units with N-oxide moieties located in the vicinity of the helical polymer backbone can be produced by noncovalent interaction with a chiral alcohol through deracemization of the biaryl pendants. The macromolecular helicity and the axial chirality induced in the PBAs are retained ("memorized") after complete removal of the chiral alcohol. Accordingly, the helical PBAs with dual static memory of the helicity and axial chirality show remarkable enantioselectivity (86% ee) for the asymmetric allylation of benzaldehyde. The enantioselectivity is slightly lower than that (96% ee) of the homochiral PBAs prepared from the corresponding enantiopure (R)- and (S)-monomers, but is comparable to that (88% ee) of the helical PBA composed of nonracemic monomers of ca. 60% ee.

Unexpectedly Strong Chiral Amplification of Chiral/Achiral and Chiral/Chiral Copolymers of Biphenylylacetylenes and Further Enhancement/Inversion and Memory of the Macromolecular Helicity.

We report an unexpectedly strong amplification of the macromolecular helicity in dynamic helical copolymers of chiral/achiral and chiral/chiral ( R/ S) biphenylylacetylenes in which the chiral residues are remote from the biphenyl pendants and further from the main chains. The copolymers consisting of 20 mol % chiral monomers and chiral monomers of 20% enantiomeric excess (ee) showed a full induced circular dichroism as intense as that of the chiral homopolymer. In contrast, an analogous poly(phenylacetylene) bearing the identical chiral residue (100% ee) showed no circular dichroism in the polymer backbone, indicating the critical role of the biphenyl moieties in the observed high chiral amplification. As anticipated, the helix-sense excesses of the copolymer backbones composed of a small amount of chiral units (<20 mol %) and chiral units of low ee (<20%) were reduced. Interestingly, however, the macromolecular helicity of the copolymers was further drastically enhanced as a greater excess of a one-handed helix or inverted upon noncovalent interaction with nonracemic alcohols and subsequently retained (memorized) after complete removal of the chiral alcohol. Even in a polymer consisting of completely racemic repeating units, one-handed right- and left-handed helices could almost be induced and memorized. These unique hierarchical amplifications and memory of the macromolecular helicity in the copolymers by the covalent and further noncovalent chiral interactions are quantitatively explained on the basis of a linear Ising model.

Chromatographic enantioseparation by poly(biphenylylacetylene) derivatives with memory of both axial chirality and macromolecular helicity.

Novel poly(biphenylylacetylene) derivatives bearing two acetyloxy groups at the 2- and 2'-positions and an alkoxycarbonyl group at the 4'-position of the biphenyl pendants (poly-Ac's) were synthesized by the polymerization of the corresponding biphenylylacetylenes using a rhodium catalyst. The obtained stereoregular (cis-transoidal) poly-Ac's folded into a predominantly one-handed helical conformation accompanied by a preferred-handed axially twisted conformation of the biphenyl pendants through noncovalent interactions with a chiral alcohol and both the induced main-chain helicity and the pendant axial chirality were maintained, that is, memorized, after complete removal of the chiral alcohol. The stability of the helicity memory of the poly-Ac's in a solution was lower than that of the analogous poly(biphenylylacetylene)s bearing two methoxymethoxy groups at the 2- and 2'-positions of the biphenyl pendants (poly-MOM's). In the solid state, however, the helicity memory of the poly-Ac's was much more stable and showed a better chiral recognition ability toward several racemates than that of the previously reported poly-MOM when used as a chiral stationary phase for high-performance liquid chromatography. In particular, the poly-Ac-based CSP with a helicity memory efficiently separated racemic benzoin derivatives into enantiomers.